A LAN or local area network is one well known system for interconnecting a large number of computer-based resources to achieve selective communication between any or all of the resources. While LANs utilize a variety of different physical interconnection arrangements and even a greater variety of communication protocols, all LANs generally use a communication medium extending between nodes of the network. The communication medium is usually the physical cabling or conductors which connect all of the nodes and over which the signals are transmitted and received. The physical medium can also include radio communication links or optical links.
Each node includes a LAN interface which is connected to the communication medium and to the computer resource at the node. In general, the function of the LAN interface is to receive signals from the communication medium, extract the information conveyed by those signals, supply that information to the computer resource, and in addition to receive information supplied by the computer resource, encapsulate that information into specific signals known as frames which are recognized by the other LAN interfaces of other nodes of the network, and to transmit the frames over the medium. The functional rules which each LAN adaptor follows in encapsulating and extracting the information, and which control the transmission, receipt and other general communication functions between the LAN interfaces at each of the nodes is referred to generally as a network protocol.
The majority of LANs utilize digital signalling for communicating between LAN interfaces. Digital signals theoretically involve the presence or absence of a voltage at specific timing intervals. The presence of a voltage at a timing interval indicates a one bit of digital information while the absence of a voltage at a timing interval indicates a zero bit of digital information. In practicality, noise and other spurious electrical influences may exist on the medium, so the presence or absence of a voltage must be distinguished from the noise in order to achieve reliable communication.
Most LANs are also subject to certain specifications regarding the distance between nodes. Depending upon the characteristics of the communication medium, significant attenuation of the signals may result over relatively long links or segments of the communication medium. Medium connectors known as hubs are frequently employed to connect the various links of cables and optical or radio paths which form the communication medium. Hubs typically include an amplifier for amplifying a signal before passing it on to other links connected to the hub. The use of the hubs throughout the communication medium have thus served to amplify the levels of the signals communicated, and this amplification resisted the natural tendency for attenuation as the signal was transmitted throughout the medium to the nodes. Furthermore, the nature of a digital signal makes it relatively easy to amplify, because amplification will simply amplify the fact that a high one bit signal level is present, while the absence of amplification maintains or even reduces the level of the noise on the medium which might otherwise detract from the proper detection of a low zero bit signal level.
Recent developments in LANs have used amplitude critical signals as a means for enhanced signalling. For example, the first four U.S. Patents mentioned above describe a new type of LAN in which multiple bits of information may be conveyed by a single phase and amplitude modulated signal transmitted over the communication medium between LAN interfaces. In order to accurately communicate the correct set of multiple bits per signal element transmitted, it is necessary to accurately detect the level of the amplitude and phase modulated signal at specific timing intervals. Failure to accurately detect the amplitude of the phase and amplitude modulated signal at the critical time will result in erroneous communication.
Another example of amplitude critical signalling, although not as critical as phase and amplitude modulated signalling, is one which involves Manchester or a similar type of coding. With Manchester coding, a one bit of information is conveyed by an abrupt signal transition within a specific timing interval, while a zero bit of information is communicated by the absence of such a transmission during a specific timing interval.
Manchester and similar coding have recently been employed to reduce radiated electromagnetic emissions from the LAN communication medium. Electromagnetic emissions occur at very high signalling frequencies, for example 50 megahertz and higher. By using Manchester or similar coding for conveying information, and by establishing multiple signal levels at which the transitions to different amplitudes may occur, the effective signalling frequency is reduced. A reduction in the effective signalling frequency reduces the amount of radiated electromagnetic radiation. For example, if the full range available for analog communications over the medium is divided into two divisions, a transition from the low level to an intermediate level would indicate a one bit, and a transition from the intermediate level to the upper level would also indicate another one bit. Arranged in this manner, the maximum radiation frequency from signals conducted over the medium would be no greater than one-fourth of the maximum transition frequency. Reduction in the radiated electromagnetic radiation by reduction in the signalling frequency is important for various health reasons and may be regulated by certain governmental agencies. However to obtain the advantage of reduced electromagnetic radiation with a Manchester or similar signalling technique, amplitude critical signals must be reliably established and maintained.
The difficulties in maintaining adequate signals in an amplitude critical signalling LAN protocol are considerably greater than those associated with maintaining signal integrity in purely digital signalling protocol. For example, the inherent attenuation created by passage of the signals through any communication medium will result in amplitude reduction of the signal at a receiver compared to the amplitude at the transmitter. However in digital signalling protocols, amplification is a relatively simple task, since any signal level greater than approximately one-half of a full range signal will be interpreted as a one bit while any signal less than approximately one-half of the full range signal will be interpreted as a zero bit. In amplitude critical signalling protocols, the amplification and decoding must be more precise.
The problems of maintaining amplitude critical signals on LANs of the type described in the above identified patents is illustrated by the characteristics of the signals. The signals can occupy any one of eight positive levels or eight negative levels. The negative level signals are actually phase shifted versions of the positive level signals. Each signal level represents a unique four bit digital signal pattern. An analog to digital converter present at each interface decodes the amplitude of the incoming signal into one of the 16 four bit patterns. It can be appreciated that if the analog level of the signal is adversely influenced from noise, other spurious signals or attenuation, for example, it may be decoded into a four bit pattern different from that which originated the signal. As a consequence, bit errors may be much more likely due to noise and attenuation. Excessive amplification may also be a problem. Too much amplification will boost the upper signal level, thereby distorting the signal and causing it to be decoded into an incorrect four bit pattern. Further still, the low level signals represented by amplitudes in the lower range must be distinguishable from the inherent noise present on the communication medium. If the amplitude of a signal falls to a sufficiently low level where the magnitude of the noise is significant relative to the magnitude of the signal, the noise has a much greater possibility of distorting the signal amplitude, again causing erroneous decoding.
As another example of the necessity to maintain the appropriate magnitude of a multilevel Manchester coded amplitude critical signal, any excessive signal amplification may cause the signals to reach the maximum level where a transition would not be distinguishable, thereby destroying the information revealed by the transitions and confusing an intended transition with an unchanged signal level.
The problems of maintaining the signal level in amplitude critical signals are compounded when the LAN signalling protocol uses a minimum of signals to encapsulate the information conveyed. For example, some types of LANs use relatively long preambles in the frame. The preamble signals constitute overhead, because they are not useful in conveying information but are only useful in achieving the intended functionality of the LAN interfaces in accordance with the network protocol. Thus, many LAN protocols attempt to increase information throughput by reducing the number of extraneous signals in each frame necessary to achieve LAN protocol functionality. Relatively lengthy preambles allow automatic gain circuits to become operative, because the length of the preamble allows an automatic gain circuit enough time to adjust to achieve a desired degree of amplification. However, relatively short preambles do not provide an adequate opportunity for automatic gain circuits to achieve adequate amplification.
Gain adjustment is even more difficult in LANs whose communication protocol involves half duplex signalling. Half duplex signalling refers to a single message transmission in a single direction or route on the LAN medium. After the one message transmission has been completed, another or a reply message is usually transmitted back in the opposite direction. Because of the singular direction and the singular occurrence of the messages, and because the messages can extend between nodes which are separated by variable and different distances over medium links having different amounts of attenuation, it is very difficult to quickly achieve the necessary gain adjustments to maintain amplitude critical signals at desired levels.
It is with respect to these problems and other problems that the present invention has evolved.